Surface mount integrated circuit devices are subject to delamination of the encapsulating plastic mold compound from the lead frame during solder attachment to a printed wiring board. In extreme cases the package cracks, which may in turn result in opens, or other reliability failures during use, or in testing. The failure mechanism has been traced to absorption of moisture by the molding compound, which vaporizes when the device is subjected to rapid heating during solder reflow processing, and the resulting vapor exerts pressure at the plastic to metal interface. This problem has intensified as packages have become larger and thinner.
In order to avoid the failure, plastic encapsulated devices which are subject to the problem must be baked relatively slowly at a temperature high enough, and for sufficient length of time to drive the moisture out of the package. Once the devices have been dried, they are sealed in a vapor barrier container, along with a desiccant for shipping and storing. The devices undergo dry baking after they have completed assembly processing and testing, and again after the containers have been opened, if they have not been used within the allowable time. Standards committees, such as EIA (Electronic Industries Association) and JEDEC (Joint Electronic Devices Engineering Council) have set specific standards related to this issue based on integrated circuit packages type, standards for testing procedures, and also standards for packing containers used for the shipping and storage.
The process of baking plastic encapsulated integrated circuits, and sealing in vapor barrier containers is generally known as dry packing. Historically, larger devices which require dry packing are placed in plastic trays with a cavity or recess sized to hold each device securely. Baking to remove absorbed water can be performed either in these trays, or the devices can be loaded into the trays immediately after dry baking. Trays loaded with dried components are placed in a seal-able vapor bag with a desiccant for shipping and storing. The trays are constructed of plastic materials which are thermally stable over the temperature range required for dry baking, typically 125 deg C. Packing trays have a number of drawbacks; namely, they are constructed of costly materials, they are bulky which contributes significantly to the shipping weight and volume, and they require space consuming automated pick and place equipment. In an attempt to minimize costs, some trays are returnable, thereby adding another expense. Automated pick and place equipment consists of large x/y tables with robot handling.
Small integrated circuit packages which require no dry packing are usually placed directly into a sprocket guided carrier tape which has a series of recesses sized to fit the devices. The recesses are set at a defined pitch, and the devices are held in place by a cover tape. The tape loaded with semiconductor devices is wound onto a packing reel, and the reel assemblage stored in a corrugated container. Tape and reel dimensions are specified by standards in order to insure uniformity between suppliers. Generally, the tape widths range from 8 mm to 56 mm and the reel is 330 millimeters in diameter. However, the reel diameter may be changed according to the volume of devices needed.
Tape and reel packaging provides a compact means for storing, transporting, and dispensing integrated circuits. The reel is placed directly onto a relatively small piece of equipment for picking and placing the circuits, and as a result this type of handling equipment has become much more desirable to the end user than the more bulky x/y table used to pick and place from trays.
Consequently, a need for packing reels and an assemblage which is compatible with dry baking integrated circuit devices has developed. Simply using existing reels constructed of high temperature plastic has not been successful because the design is inefficiency for baking, and because high temperature plastics are generally more dense, thus resulting in additional shipping weight.